Semiconductor layer

ABSTRACT

The present invention provides a highly doped semiconductor layer.

FIELD OF THE INVENTION

[0001] The present invention relates generally to the field ofsemiconductors and more particularly to a highly conductivesemiconductor layer having two or more impurities.

BACKGROUND OF THE INVENTION

[0002] Semiconductors, either single element or compound (e.g., III-V)semiconductors, are widely used in integrated circuits. For example,Gallium Arsenide (GaAs) semiconductors are widely used in low-noise,high-gain, weak-signal amplifying devices. The useful properties ofsemiconductors depend not only on the particular semiconductor thatforms the crystal, but also on the dopants that are incorporated intothe crystal lattice.

[0003] Semiconductor devices require the use of highly conductivelayers. The use of impurities or dopants contributes to the hole andelectron charge carriers that are responsible for the electronicproperties of the crystals. When excess electrons are generated, theimpurity is considered a “donor” or “n-type” dopant. When excess holesare generated, the impurity is considered an “acceptor” or “p-type”dopant.

[0004] More specifically, depending on the valence of the impurity andthe lattice site, the conductivity of a semiconductor layer depends upon(i) the number of electrons (or holes); (ii) the electron (or hole)mobility; and (iii) the charge of the electron. Thus, there is a directcorrelation between the level of impurity and the conductivity of thesemiconductor layer. Thus, in order to create a highly conductive layer,the density of the impurity must also be very high.

[0005] Importantly, however, when any single impurity is added in veryhigh concentrations in an attempt to maximize conductivity, degradationproblems occur. Such problems depend on the particular dopant used, andinclude but are not limited to auto compensation, diffusion, strain, andother defects. For example, when a Beryllium (Be) dopant is used above aparticular concentration, diffusion may occur that degrades deviceperformance. As another example, excessive use of a Carbon (C) dopantlikely causes auto-compensation that degrades device performance.Throughout this specification, the term “degradation concentration” isused to describe the concentration at which a particular impurity beginsto create detrimental effects or degrade semiconductor performance. Inother words, each impurity currently used to create conducting layershas some maximum acceptable concentration before the layer is degradedin some manner. See, for example, Doping in Semiconductors, E. F.Schubert, University Press, 1993, herein incorporated by reference.

[0006] To date, attempts to reach beyond the accepted degradationconcentration for particular dopants have focused on manufacturingtechniques to reduce the detrimental effects. For example, attempts toovercome the degradation concentration for Be have focused on reducingthe layer temperature during manufacture. As another example, to reduceauto-compensation, strict control is maintained on the ratio of thesemiconductor elements. Such techniques often add to the manufacturingtime and costs, while providing only marginal gains in the conductivityof the semiconductor layer. Furthermore, such techniques focus onminimizing rather than avoiding or eliminating the potential detrimentaleffects.

[0007] There is a need in the art for a highly conductive semiconductorwithout the aforementioned degradation problems.

SUMMARY OF THE INVENTION

[0008] The present invention provides a novel approach to the creationof a highly conductive semiconductor layer. As noted above, priorattempts to create highly conductive layers have focused on minimizingrather than eliminating the potential detrimental effects. The presentinvention eliminates the detrimental effects of particular impurities byavoiding the use of individual impurities at densities beyond theirrespective degradation concentrations. The present invention combinestwo or more dopants, each at a level below the dopant's degradationconcentration, to provide a highly conductive layer.

[0009] More specifically, the present invention provides a semiconductorlayer that includes at least two impurities. Each impurity is introducedat a level below its respective degradation concentration. In thismanner, the two or more impurities provide an additive conductivity tothe semiconductor layer at a level above the conductivity possible withany one of the impurities alone, due to the detrimental effects thatwould be created by increasing the concentration of any one impuritybeyond its degradation concentration.

[0010] An additional aspect of the present invention is a semiconductorlayer having two or more epitaxially grown impurities of the samecarrier type. Preferably, the two or more impurities each has a smallercovalent radius than the layer atoms. More preferably, the two or moreimpurities are grown at substantially equivalent concentrations. Thepresent invention also provides a method for creating a semiconductorlayer having two or more impurities introduced during layer formation.

[0011] These and other aspects of the present invention as disclosedherein will become apparent to those skilled in the art after a readingof the following description of the preferred embodiments whenconsidered with the drawings. The drawings are for the purpose ofdescribing a preferred embodiment of the invention and are not intendedto limit the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawing figures incorporated in and forming apart of this specification illustrate several aspects of the invention,and together with the description serve to explain the principles of theinvention.

[0013]FIG. 1 is a cross-sectional view of typical layers in aheterojunction bipolar transistor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] The embodiments set forth below represent the necessaryinformation to enable those skilled in the art to practice the inventionand illustrate the best mode of practicing the invention. Upon readingthe following description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the inventionand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

[0015]FIG. 1 illustrates a cross-sectional view of an NPN-typeheterojunction bipolar transistor (“HBT”). As illustrated, the HBTgenerally includes an emitter layer 10, base layer 12, and collectorlayer 14. As those skilled in the art will appreciate, the HBT will alsoinclude a subcollector layer 16 underneath the collector layer 14 and asemi-insulating substrate layer 18 underneath the subcollector layer 16.Further, to facilitate contact with the emitter layer 10, base layer 12,and collector layer 14, an emitter contact layer 20 topped with emittermetal 22 is provided to facilitate electrical contact with the emitterlayer 10. Base metal 24 on top of base layer 12 facilitates electricalcontact to the base layer 12 at multiple locations. Similarly, collectormetal 26 is provided on subcollector layer 16 to facilitate electricalcontact to the collector layer 14 and subcollector layer 16 at multiplelocations.

[0016] As discussed above, an NPN-type HBT requires a highly conductivep-type base layer 12. Generally, the conductivity of a semiconductorlayer is proportional to doping. The present invention provides a highlyconductive, heavily doped, epitaxial layer. The present invention dopesa semiconductor layer by introducing two or more impurities during thelayer formation. Thus, as opposed to manufacturing techniques thatdiffuse or implant a dopant after formation of the layer, the presentinvention introduces at least two dopants during layer formation.Although any acceptable layer-formation technique should be consideredwithin the scope of the present invention, the most preferred techniqueis molecular beam epitaxy, as is known in the art.

[0017] The present invention includes the introduction of two or moredopants, such as both Beryllium (Be) and Carbon (C), to a semiconductorlayer, such as a Gallium Arsenide (Gas) or other III-IV compoundsemiconductor layer. Heretofore, Be could be used at densities up toapproximately 1.5×10¹⁹ cm⁻³ before diffusion of the Be atoms degradesdevice performance. Thus, Be has a degradation concentration ofapproximately 1.5×10¹⁹ cm⁻³. Likewise, C may be used at densities ofapproximately 1.5×10¹⁹ cm⁻³ before auto compensation degrades deviceperformance. Thus, C has a degradation concentration of approximately1.5×10¹⁹ cm⁻³ as well. Importantly, although Be and C have similardegradation concentrations, each dopant has a particular degradationconcentration based upon the individual characteristics of the dopant.As noted above, reference is made to Doping in Semiconductors, E. F.Schubert, University Press, 1993, herein incorporated by reference, fora relatively comprehensive collection of dopants and respectivedegradation concentrations. The scope of the present invention isintended to cover all dopants, each having a particular degradationconcentration.

[0018] Although the present invention is believed applicable to anyhighly doped layer, preferably, the two or more impurities are the samecarrier type. Thus, preferably, the present invention provides twoacceptor dopants, occupying both cation and anion sites of the crystallattice. Additionally, the covalent radii of the impurities need notoffset one another. In other words, each impurity used in the presentinvention may have a smaller covalent radius to the layer atoms, suchas, for example both Be and C having smaller covalent radii than the Gaand As covalent radii.

[0019] As noted above, a preferred method for forming a layer of thepresent invention is through molecular beam epitaxy (“MBE”). As is knownin the art, MBE is a deposition technique performed in ultra high vacuumto grow compound semiconductors. In MBE, atoms of an element or compoundare delivered to a substrate through an ultra-pure, ultra-high vacuum(“UHV”) atmosphere. The UHV atmosphere provided by the MBE chamberminimizes impurities and allows the atoms to arrive on the substratewithout colliding with other atoms or molecules, thereby minimizingcontaminants. The heated substrate surface allows the arriving atoms todistribute themselves evenly across the surface to form the crystalstructure.

[0020] In MBE, the substrate is placed in an UHV chamber with directline of sight to several elemental species, each of which is in anevaporation furnace commonly referred to as an effusion cell. As isknown, through the use of shutters and through control of the effusioncell temperatures, a variety of material composition and doping can beachieved. Notably, however, heretofore the advantages of combining twoor more dopants have not been recognized. Thus, the present inventionincludes the introduction of dopants during the growth of the crystallayer structure.

[0021] For example, as noted above, Be and C have degradationconcentrations of approximately 1.5×10¹⁹ cm⁻³. Thus, during layerformation, such as during MBE, approximately 1.5×10¹⁹ cm⁻³ of Be andapproximately 1.5×10¹⁹ cm⁻³ of C are introduced as impurities to thecrystal structure. As shown in the table below, the combination of Beand C as dopants provides a resulting layer having properties that atleast reaches, but often exceeds, the beneficial properties availablewith either dopant alone. Doping Level Thickness Resistance B (cm⁻³)Dopant (Å) (Ω) (1 mA) 3 × 10¹⁹ Be 800 324 197 3 × 10¹⁹ C 800 320 77 3 ×10¹⁹ ½ C and ½ Be 800 327 153

[0022] As noted above, the doping levels for the structures including Beonly and C only far exceed the degradation concentration for eachindividual dopant. Thus, although the Be only structure appears to havethe resistance and high β desired, such a structure has severereliability problems due to Be diffusion. Notably, the C only dopedstructure illustrates the problem of carbon clustering, namely, areduced β.

[0023] On the other hand, the present invention provides a more highlyconductive layer than heretofore possible. As noted above, theconductivity of a semiconductor layer is the sum of the electron andhole contributions. To illustrate:

σ=neμ _(e) +peμ _(h);

[0024] where n and p each represent, respectively, the concentrations ofelectrons and holes, e represents the charge of an electron (1.6×10⁻¹⁹C), and each μ represents the mobility of the electrons (e) and holes(h), respectively. Using the present example, the intrinsic carrierconcentration (n_(i)) for GaAs is 1.8×10⁶ cm⁻³. As is appreciated in theart, the concentrations of electrons and holes are related to theintrinsic concentration through the equation (n*p=n_(i) ²). Therefore,only at very high doping concentrations will the majority of the layerconductivity result from the intentional dopant.

[0025] In the present example, with high p-type doping the neμ_(e)portion of conductivity is approximately zero (0), and the resistivity(R) of the heavily doped p-type material can be estimated sinceresistivity (R) is the inverse of conductivity (σ). As is known, thenumber of holes in the layer p is equal to the dopant density (cm⁻³)multiplied by the layer thickness. Therefore, since the presentinvention provides for an 800 layer to be doped at 3×10¹⁹ cm⁻³ with amobility of 80 cm²/V sec the resistivity is:

R=1/σ

R=1/(neμ _(e) +peμ _(h))

R=1/peμ_(h)

R=1/[(3×10¹⁹)*(800×10⁻⁸)*(1.6×10⁻¹⁹)*(80)]

R=327

[0026] Thus, the present invention provides for superlative resistancewhile avoiding degradation problems associated with the individualdopants used.

[0027] Although the above example includes two dopants, the scope of thepresent invention encompasses any number of impurities, providedhowever, that no dopant is used at such a level as to degrade thesemiconductor. Thus, a plurality of impurities may be added, providedthat no impurity is added in a concentration substantially above thedegradation concentration for the particular impurity.

[0028] Moreover, the use of two or more impurities often demonstratessynergistic effects. For example, with the structure formed with Be andC, the presence of C appears to reduce Be diffusion. Similarly, thepresence of Be appears to reduce C clustering. Thus, the concentrationof impurities with the present invention is greater than the sum of theindividual degradation concentrations.

[0029] The highly conductive semiconductor layers of the presentinvention are believed useful in a variety of applications, for example,without limitation, in optoelectronics and transistors, such as HBTs,solar cells, LEDs, LASERs, and FETs. The invention is also applicable tomagnetoresistors.

[0030] Although specific embodiments of the present invention have beenillustrated and described in detail, it is to be expressly understoodthat the invention is not limited thereto. The above detaileddescription of the embodiment is provided for example only and shouldnot be construed as constituting any limitation of the invention.Modifications will be obvious to those skilled in the art, and allmodifications that do not depart from the spirit of the invention areintended to be included within the scope of the appended claims.

What is claimed is:
 1. A semiconductor layer comprising: a firstimpurity, said first impurity having a first degradation concentration;and a second impurity, said second impurity having a second degradationconcentration, whereby each of said first impurity and second impurityare introduced in the semiconductor layer at concentrations below therespective first and second degradation concentrations.
 2. Thesemiconductor layer of claim 1 wherein the first and second impuritiesare the same carrier type.
 3. The semiconductor layer of claim 1 whereinthe semiconductor layer is epitaxially grown.
 4. The semiconductor layerof claim 1 wherein each of said first and second impurities has asmaller covalent radius than the layer atoms.
 5. The semiconductor layerof claim 1 further comprising a third impurity, said third impurityhaving a third degradation concentration, and said third impurity beingintroduced at a concentration below said third degradationconcentration.
 6. The semiconductor layer of claim 1 wherein thesemiconductor layer is GaAs.
 7. The semiconductor layer of claim 1wherein the first impurity is Be.
 8. The semiconductor layer of claim 7wherein the second impurity is C.
 9. The semiconductor layer of claim 1wherein the second impurity is C.
 10. The semiconductor layer of claim 1wherein the first and second degradation concentrations areapproximately 1.5×10¹⁹ cm⁻³.
 11. The semiconductor layer of claim 1wherein the first and second impurities are introduced at a combinedconcentration greater than the sum of the respective degradationconcentrations.
 12. The semiconductor layer of claim 1 wherein the layeris used in an optoelectronic device or a transistor.
 13. A semiconductorlayer comprising: two or more epitaxially grown impurities of the samecarrier type.
 14. The semiconductor layer of claim 13 wherein the two ormore impurities each has a smaller covalent radius than the layer atoms.15. The semiconductor layer of claim 13 wherein the two or moreimpurities are grown at substantially equivalent concentrations.
 16. Asemiconductor layer comprising: a first dopant, having a concentrationbelow a degradation concentration; and a second dopant, having aconcentration below the degradation concentration, wherein thesemiconductor layer has a total dopant concentration of at leastapproximately 3×10¹⁹ cm⁻³.
 17. The semiconductor layer of claim 16wherein the semiconductor layer is epitaxially grown.
 18. Thesemiconductor layer of claim 16 wherein the layer is GaAs.
 19. Thesemiconductor layer of claim 16 wherein the first dopant is Be.
 20. Thesemiconductor layer of claim 19 wherein the second dopant is C.
 21. Thesemiconductor layer of claim 16 wherein the second dopant is C.
 22. Thesemiconductor layer of claim 21 wherein the degradation concentrationsof the first and second dopants are approximately 1.5×10¹⁹ cm⁻³.
 23. Thesemiconductor layer of claim 16 further comprising one or moreadditional dopants, each of said additional dopants having a degradationconcentration, and each additional dopant provided at a respectiveconcentration below the respective degradation concentration for thatdopant.
 24. A method for forming a semiconductor crystal layercomprising: a. introducing a first impurity during formation of thecrystal layer; b. introducing at least a second impurity duringformation of the crystal layer.
 25. The method of claim 24 furthercomprising introducing at least a third impurity to the crystal layer.26. The method of claim 24 wherein the crystal layer is GaAs.
 27. Themethod of claim 24 wherein the first impurity is Be.
 28. The method ofclaim 27 wherein the second impurity is C.
 29. The method of claim 24wherein the at least a second impurity is C.
 30. The method of claim 24wherein the first impurity and the second impurity are introduced atconcentrations of approximately 1.5×10¹⁹ cm⁻³.
 31. The method of claim24 wherein the at least two impurities are introduced epitaxially. 32.The method of claim 31 wherein the at least two impurities areintroduced by molecular beam epitaxy.
 33. The method of claim 24 whereineach of said first impurity and said second impurity is introduced inthe semiconductor crystal layer at concentrations below the respectivefirst and second degradation concentrations.